Solid-state image sensors, also known as imagers, were developed in the late 1960s and early 1970s primarily for television image acquisition, transmission, and display. An imager absorbs incident radiation of a particular wavelength (such as optical photons, x-rays, or the like) and generates an electrical signal corresponding to the absorbed radiation. There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices (CIDs), hybrid focal plan arrays, and CMOS imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems and data compression systems for high-definition television, among other uses.
These imagers typically consist of an array of pixel cells containing photosensors, where each pixel produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be stored, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image. The photosensors are typically phototransistors, photoconductors or photodiodes. The magnitude of the signal produced by each pixel, therefore, is proportional to the amount of light impinging on the photosensor.
To allow the photosensors to capture a color image, the photosensors must be able to separately detect red (R) photons, green (G) photons and blue (B) photons. Accordingly, each pixel must be sensitive only to one color or spectral band. For this, a color filter array (CFA) is typically placed in front of the pixels so that each pixel measures the light of the color of its associated filter. Thus, each pixel of a color image sensor is covered with either a red, green or blue filter, according to a specific pattern.
FIG. 1 illustrates one such color filter array pattern, known as the “Bayer” pattern, which is described in more detail in U.S. Pat. No. 3,971,065 (the disclosure of which is incorporated by reference herein). In the Bayer pattern, red, green and blue pixels are arranged so that alternating pixels of red and green are on a first row of an image, and alternating pixels of blue and green are on a next row. Thus, when the image sensor is read out, line by line, the pixel sequence for the first line reads GRGRGR etc., and then the alternate line sequence reads BGBGBG etc. This output is called sequential RGB or sRGB.
In the Bayer pattern, sampling rates for all three basic color vectors are adjusted according to the acuity of the human visual system. That is, green color, to which the human eye is most sensitive and responsive, is sampled most frequently, whereas blue color, for which the human vision has least resolution, is sampled the least frequently. This is why in the Bayer pattern, the green-sensitive elements, which serve to detect luminance (the color vector which provides the luminance information) occur at every other array position, while the red-sensitive elements alternate with the blue-sensitive elements.
As a result of these attributes, the Bayer pattern has vast applications in imaging objects having a more or less uniform representations of colors across the entire visible spectrum. Thus, sampling the green color at twice the frequency of the other primary colors provides a good representation of the luminance component of a particular object being imaged. Nevertheless, if the object being imaged has a relatively low spectral reflectivity in the green part of the wavelength, the image captured with an imager employing a Bayer color filter pattern can be suboptimal.
There is needed, therefore, a color filter array pattern of a CMOS-sensor for sensing objects which do not have a uniform representation of colors across the visible spectrum, for example, elements of the human body non-visible to the naked eye, such as the internal organs of the gastrointestinal tract. A method of fabricating such color filter pattern is also needed.